1. Field of the Invention
The present invention relates to a rotary electric machine applied to a motor for an electric power steering assembly for assisting, for example, the steering force of a steering wheel of a vehicle.
2. Description of the Related Art
FIG. 55 is a side sectional view of a conventional motor (hereinafter, referred to as an electric motor) 100 used in an electric power steering assembly. The electric motor 100 includes a cylindrical yoke 1, two field permanent magnets 2, which are fixed in the yoke 1 in confrontation with each other and disposed in the circumferential direction of the yoke 1, a shaft 4 rotatably disposed in the yoke 1 through a bearing 3, an armature 5 fixed to the shaft 4, a commutator 6, which is fixed to an end of the shaft 4 and composed of a plurality of copper segments 16, and brushes 8 abutted against the surface of the commutator 6 by the elastic force of a spring 7.
The armature 5 includes a core 9 having a plurality of slots 11 extending in an axial direction and a winding 10 having a conductive wire wound around the slots 11 by a lap-winding method.
In the electric motor 100, which employs the lap-winding method and has 2 poles, a current is supplied to the winding 10 from the outside through the brushes 8 abutted against the segments 16 to cause the armature 5 to rotate together with the shaft 4 by an electromagnetic action.
Since the electric motor 100 is mainly used in vehicles of comparatively light weight having a small amount of displacement, it has a small amount of assist torque, thus its operation noise is very low and is not almost felt even in a passenger compartment.
Incidentally, employment of a power steering assembly using a DC motor in place of a hydraulic power steering assembly is started to reduce fuel consumption and to decrease weight even in heavy vehicles having a medium and large amount of displacement to cope with social requirements for a saving in fuel and a reduction in exhaust gases. In this case, while an electric motor having a large amount of torque is necessary, when the motor is designed maintaining 2 poles and the lap-winding method, a size of the motor is increased. Thus, it is necessary to design a motor having multi-poles such as 4 poles to reduce the size of the motor and to generate a large amount of torque.
FIGS. 56 and 57 are views for comparing a DC motor having 2 poles and 14 slots (hereinafter, abbreviated as a 2-pole motor) and a DC motor having 4 poles and 21 slots (hereinafter, abbreviated as a 4-pole motor) as an example of a multi-pole motor. In the figures, the inventors determined a difference of magnetic attracting forces acting on the armatures of the 2-pole motor and the 4-pole motor by a field analysis when the armatures were operated at decentered positions. In FIG. 56, a symbol xe2x80x9cxe2x80xa2xe2x80x9d shows a center of a stator, that is, an intrinsic center of rotation, and a symbol xe2x80x9cxxe2x80x9d shows a center of rotation in a decentered state. Then, as apparent from the above figures, it can be found that the 4-pole motor is more liable to generate oscillation and noise than the 2-pole motor.
That is, a force acting on each armature was examined when each armature was decentered from the intrinsic center thereof as a start point by the same decentering amount (decentering amount: 0.1 mm) in the respective angles in a decentering direction of from 0xc2x0 to 360xc2x0. As a result, it was found that an electromagnetic attracting force of about 0.45 N acted on the 2-pole motor at a maximum in a decentering direction, whereas an electromagnetic attracting force of about 2.7 N (6 times that of the 2-pole motor) acted on the 4-pole motor at a maximum in a decentering direction. Remarkable directionality of the magnetic attracting force due to decentering is found in the 2-pole motor. When electromagnetic attracting forces in a decentering direction are compared as to a case in which the armature of the 2-pole motor is decentered in an inter-pole direction (angles in the decentering direction are 90xc2x0 and 270xc2x0) and a case in which it is decentered in a pole center direction (angles in the decentering direction is 0 and 180xc2x0), a magnetic attaching force, which is twice that of the latter case, acts in the former case. In contrast, no remarkable directionality is found in the 4-pole motor. That is, a magnetic attracting force in a decentering direction is about 2.7 N at all the angles in the decentering direction of the from 0xc2x0 to 360xc2x0, which means that xe2x80x9cthere exists a safe direction with respect to decentering in the 2-pole motor but there does not exist a safe direction in the 4-pole motorxe2x80x9d. It is contemplated that this difference relates to the above difference in the generation of oscillation and noise.
While it is necessary to achieve the multi-pole such as the 4-pole to design a motor having a reduced size and an increased amount of torque, there remains the problem of oscillation and noise.
Incidentally, there is contemplated, for example, an armature employing a single wave winding method, in addition to an armature employing the lap-winding method when the multi-pole is achieved to cope with a reduction in size and an increase of torque. While brushes as many as poles are ordinarily provided in the lap-winding, 2 brushes are generally provided in the single wave winding.
FIGS. 58 and 59 are views showing an electromagnetic attracting forces acting on armatures having 4 poles and 21 slots as an example of the multi-pole, wherein FIG. 58 shows a case employing a lap-winding and 4-brush method and FIG. 59 shows a case employing a single wave winding and 2-brush method.
In the comparison of both the figures, when an armature is rotated by 1 slot, a magnetic attracting force acts on the armature in a radially external direction at all times as shown by arrows xe2x80x9caxe2x80x9d when the armature is of a single wave winding type, whereas when an armature is of a lap-winding type having the 21 slots, a magnetic attracting force acts thereon in a direction which is varied in a circumferential direction as shown by arrows xe2x80x9cbxe2x80x9d. Thus, there is a problem that the armature of the lap-winding type having the 21 slots is liable to generate oscillation due to rotation and liable to generate noise accordingly.
Further, when an armature has multi-poles and an odd number of brushes and employs the lap-winding method, since there are caused differences in induced voltage between circuits of the winding of the armature by the influences of decentering of the armature, uneven currents flowing through brushes, machining errors and the like, circulating currents, which run through the brushes, are generated in the armature. As a result, there also arises a problem of an increase in operation noise due to increased temperatures of the brushes and a commutator, reduced lives thereof, and an increase in a torque rip, which are accompanied by deterioration of a rectifying action and an increase in rectifying sparks generated from the brushes, and the combined actions thereof.
In contrast, when an armature has multi-poles and an odd number of slots and employs the single wave winding method, there is a problem that the torque ripple is increased (the torque ripple is 0.096% in the lap-winding method of FIG. 58, whereas it is 1.37% in the single wave winding method of FIG. 59.).
An object of the present invention, which was made to solve the above problems, is to provide a rotary electric machine capable of lowering operation noise or the like.
To this end, according to the present invention, there is provided a rotary electric machine, comprising: a yoke; a multi-polar magnetic field portion composed of 4 poles fixed to the inner wall of said yoke; a shaft disposed within the yoke so as to be able to rotate freely; an armature fixed to the shaft having a winding composed of a conductor wire wound by double wave winding into an even number of slots formed on the outer circumferential surface of a core so as to extend in the axial direction thereof; a commutator comprising a plurality of segments fixed to an end portion of the shaft; and a plurality of brushes contacting the surface of the commutator.
According to another aspect of the present invention, there is provided a rotary electric machine, comprising: a yoke; a multi-polar magnetic field portion composed of 4 poles fixed to the inner wall of the yoke; a shaft disposed within the yoke so as to be able to rotate freely; an armature fixed to the shaft having a winding composed of a conductor wire wound by double wave winding into a number of slots being an integer multiple of the number of pairs of the poles, the slots being formed on the outer circumferential surface of a core so as to extend in the axial direction thereof; a commutator comprising a plurality of segments fixed to an end portion of the shaft; and a plurality of brushes contacting the surface of the commutator.
According to still another aspect of the present invention, there is provided a rotary electric machine comprising: a yoke; a multi-polar magnetic field portion composed of at least 6 poles fixed to the inner wall of the yoke; a shaft disposed within the yoke so as to be able to rotate freely; an armature fixed to the shaft having a winding composed of a conductor wire wound by multiple wave winding into a number of slots being an integer multiple of the number of pairs of the poles and being not an integer multiple of the number of the poles, the slots being formed on the outer circumferential surface of a core so as to extend in the axial direction thereof; a commutator comprising a plurality of segments fixed to an end portion of the shaft; and a plurality of brushes contacting the surface of the commutator.